1. Field of the Invention
This invention relates to the field of electronic cable equipment, and more specifically, to cables used for the concurrent transmission of analog and digital signals, such as for analog video and digital audio.
2. Background Art
Data networks, or LANs, typically use low cost UTP (unshielded twisted pair) cables for bi-directional communication of digital data. In addition, UTP may be used to convey analog video signals over a dedicated video-type network. In both cases, the current constructions of UTP wherein four twisted pairs are utilized, involve manipulation of the twisted pairs such that each pair has a different lay length (i.e., the length of a single twist interval) throughout the cable so as to minimize cross-talk of data signals between pairs. Various lay lengths are combined such that coupling is minimized.
FIGS. 1A and 1B illustrate the construction of a prior art Unshielded Twisted Pair (UTP) cable for data transmission. As illustrated in FIG. 1A, a typical UTP cable 100 comprises four twisted pair wires 104, 106, 108, and 110, all located within a cable bundle. The bundled twisted pair wires are held together with insulation layer 102. Referring to FIG. 1B, each of the four twisted pairs (e.g. 104, 106, 108 or 110) consists of two wires identified with suffix “A” and “B” and having a specific lay length. For instance, twisted pair 104 comprises wire 104A and wire 104B having a constant lay length “A” throughout its length; twisted pair 106 comprises wire 106A and wire 106B having a constant lay length “B” throughout its length; twisted pair 108 comprises wire 108A and wire 108B having a constant lay length “C” throughout its length; and twisted pair 110 comprises wire 110A and wire 110B having a constant lay length “D” throughout its length.
As illustrated, each of the prior art twisted pair cables (e.g. 104, 106, 108, or 110) has a specific lay length different from the other twisted pairs. All of these twisted pairs, each one made with a specific lay length, are located side-by-side within a cable bundle. The different lay lengths contribute to reduced crosstalk.
In the application of analog video, including RGB analog video or graphics, UTP data cables may be utilized. Implementations suffer from the fact that the data cable construction having different lay lengths between conducting pairs results in each pair having a different electrical length. The differing electrical lengths result in proportional delay of the video signal when applied over the long distances (around 100 meters or more) typically encountered in this type of application.
The different electrical lengths result in a relative delay between RGB signals in an RGB analog video implementation, for example. The delay period is long enough to create an offset of visual information on the display screen so as to appear misconverged. Graphics details will not properly line up on the screen at the appropriate location. This “fringing effect” makes for poor quality or totally unacceptable video performance. To counteract this affect, some form of delay must be added to the shorter pathways in the transmission line to equalize the delays such that the longest delay becomes the standard by which the others are adjusted. Various methodologies for accomplishing this are known. For example, an appropriate length of cable may be added to each of the faster transmission lines to compensate. In addition, various electrical circuit schemes exist for delaying video channels within the processing system that receives the UTP-transmitted information and converts it to usable analog content.
The above scenario describes results based on use of three of the four available twisted pairs within the bundle. The three active pairs are conveying red, green, and blue signals, respectively. The fourth pair of the bundle may or may not be used. In cases where the fourth pair is used, it may carry digital control and/or audio channels. For example, stereo audio may be digitized and packetized for transmission over the fourth pair, allowing for simultaneous distribution of stereo audio along with RGB analog video or computer graphics. The intimacy of this fourth pair with the other three carrying video information can cause significant signal coupling, or crosstalk, wherein the digital signal induces noise into any or all of the three video-carrying pairs. This crosstalk may take many forms. Among those forms may be high frequency random noise effects (sometimes called “popcorn noise”) or differentiated signal edges induced into the analog video information and erroneously interpreted by the target display device as normal synchronization pulses. This latter condition may falsely trigger the display, which typically causes the display to blank the display screen.
Extron recognized a need to correct for these problems by manufacturing a low-skew UTP cable. In the low-skew cable, the lay lengths of the twisted pairs are equal in length. Equal lay lengths on the pairs mean that the electrical lengths are very nearly equal. The time difference becomes so small that it is, for all practical purposes, negligible. Furthermore, the twisted pairs are bundled together utilizing the standard twist-lay process used for such a cable. This is a departure for UTP-type data networks because the equal length pairs will promote close coupling of digital data and not be suitable for data networks.
In analog video and graphics application, the cross coupling is not a prime issue. The cross coupling is small enough that the receiver can be equalized to mostly ignore it since the analog video system is a one-way transmission application. However, when digital control signals and/or digital audio channels are conveyed over the fourth pair in these applications, noise is often induced into one or more of the video-carrying pairs. Additionally, the signaling voltage on the typical analog RGB or video system is not compliant with voltages used for data networks. This means that UTP cabling that might be used for data networks must be wholly dedicated to the analog video application and cannot be shared. Applying the typical analog video connection to a UTP within a data network will not only be format incompatible, it will likely damage network components.
Utilization of the low-skew UTP cable is appropriate for dedicated installations where prior knowledge of the analog video/graphics system is prescribed. Clearly, this cable will be dedicated to the analog “network” and not used for data. Likewise, the data network could use the cable but will likely not use it because key crosstalk parameters important to data network communications will be severely compromised such that the data network node may not perform at all.
Therefore, there is a need for a cable that can satisfy the low skew requirements of video signaling, while providing sufficient cross-talk isolation so that digital information simultaneously conveyed through the cable does not significantly impair the quality of the video signals.